Composition and process for electrolytic cleaning of metals



Patented Apr. 3, 1951 OFFICE CQMPOSITIQN AND PROCESS. FOR ELEC- TROLYTIC CLEANING OF METALS Hugh G. Vlhehster,v Detroit, Mich., assignor to r J. H. Shoemaker, Detroit, Mich.

No. Drawing. Application January 8, 1949,

Serial No. 69,988

Qlaims. (Cl. 204-145) This application relates to the art of treating metals and particularly to the art of cleaning work pieces of metal. Any metal which in itself is not subject to rapid attack by the bath hereof may be usefully cleaned of oxides and scale. The greatest utility is in the treatment of ferrous metals, typically iron of various carbon contents and which may contain typical alloying inradients.

A prior application, Serial No. 34,812 of June 23, 1948, now Patent 2,468,006 issued April 19, 1949 discloses a metal cleaning process, which may briefly be described as follows. A work piece Which is coated with scaleand oxide and is to be cleaned is immersed irra fused molten salt bath consisting of about 85% sodium hydroxide which may vary in the approximate range of '75 to 90' parts by weight, about sodium chloride which may vary in the approximate range of 7 to 13 parts by weight, and at least one-half of 1% of sodium aluminate which may vary in the approximate range of .5 to 5 parts by weight. Preferably, but not necessarily, there will be about 4% sodium fluoride which may vary in the approximate range of 2 to 6 parts by weight, but if this ingredient is not thought desirable, it may be omitted and the percentage of the other ingredients correspondingly increased slightly. The fluoride is generally unnecessary Where the work piece is not covered with scale containing sand. The time of immersion may be as long as necessary to obtain complete cleaning. The temperature of the bath may be anywhere above the melting point, approximately 600 F. and below the vapor point, approximately 1000 F., but excellent results have been obtained in the vicinity of 850 to 900 F. A reversing direct current is applied to the bath through the work piece as described in the prior application. Work pieces so immersed and so treated electrically will be cleaned by such bath.

In the commercial use of such a bath, several problems have arisen and in this application, solutions for these problemsare disclosed.

First, inmany instances, stains have appeared on the work pieces cleaned by the bath. Secondly, in the use of the bath, the dissolved scale impairs the functioning and efficiency of the bath.

According to the present invention it has been found that the addition of a stable inorganic cyanide to the bath has proven to be effective in eliminating the formation of stains on metals cleaned in the bath andcauses the transformation of the dissolved scale into an insoluble compound which precipitates in the bath, no longer being soluble in it, anddrops to the bottom of the potto form a sludge which can be removed mechanically.

The cyanides found to be useful herein are any stable inorganic cyanides soluble in the fused bath and of a'character such as to be stable in the substantial absence of oxidizing agents. Alkali and alkali earth. metal cyanides are typically used such as potassium, sodium and calcium, such being preferred not only for reasons of effectiveness but also economic availability and inexpensiveness. Of these in general sodium cyanide is preferred.

The improvement of this application to baths such as those of the priorapplication and other molten alkali salt baths is in the addition of cyanide. The cyanide is added in minor quantity to a major bath component typically comprising fused caustic soda which usually contains other modifying ingredients such as sodium chloride, sodium carbonate, sodium fluoride and sodium aluminate, as are known in the art and typically shown as a bath in my co-pending application. Such bath contains a predominant quantity, usually exceeding of the bath of caustic and minor quantities of the other components. While oxidizing agents have been used in various baths, such oxidation agent will tend to destroy the cyanide by oxidation. Hence, baths of the present invention will generally contain, if any, less oxidizing agent than would destroy cyanide by oxidation thereof, and in any case less than 5%. The cyanide used with the bath is in minor quantity sufficient to reduce the ferric oxide stains, generally greater than .5% but usually not more than 10%. It will be understood that since the cyanide is oxidized in the course of cleaning the metal, further cyanide may be added as needed.

The preferred sodium cyanide comes commercially in the form of round eggs or lumps of approximately one ounce each. A handful, fifteen or twenty of them, may be dumped into the bath and the cyanide eggs dissolve. The amount of cyanide that is used may be determined by analysis of the bath and of the stains or impurities on the metal being cleaned, or it may b determined experimentally, by trial and error. An experienced operator will learn very quickly how much cyanide to add to overcome the problem of staining and the problem of the impairing of efficiency resulting from the dissolved scale.

A bath made by fusing a mixture of chemicals having the following composition in parts by weight is given as a typical example:

Parts by weight It is believed that the stains are in the form of oxides which are caused by oxidizing members in the bath. The cyanide seems to function as a reducing agent for the oxidizing members and thus removes the cause of the stains.

It is also believed that the dissolved scale is in the form of an iron oxide which is soluble in the bath. The cyanide seems to transform the soluble oxide into an insoluble oxide which precipitates in the bath, dropping as a sludge.

It is pointed out here that the oxidation agents which cause staining are formed in the bath in commercial practice in several different ways. One way is by mistake informula, or improper addition of oxidation members to the bath. Another way is from the dissolving of oxidation agents off the work piece, an example being manganese dioxide dissolving off a cast iron work piece.

I claim:

1. For electrolytically cleaning metal, a composition comprising 75 to 90 percent by Weight of caustic alkali, 7 to 13 percent by'weight of sodium chloride, 0.5 to percent by weight of sodium aluminate and 0.5 to 10 percent by weight of alkali cyanide.

2. For electrolytically cleaning metal, a composition comprising 75 to 90 percent by weight of caustic alkali, 7 to 13 percent by weight of sodium chloride, 2 to 6 percent by weight of sodium fluoride, 0.5 to 5 percent by weight sodium aluminate and 0.5 to 10 percent by weight of alkali cyanide.

3. For electrolytically cleaning metal, a composition consisting essentially in approximate proportions of percent by weightof caustic soda, 10 percent by weight of sodium chloride, 4 percent by weight of sodium fluoride, 1 percent by weight of sodium aluminate and 0.5 to 10 percent by weight of sodium cyanide.

4. For electrolytically cleaning metal, a composition consisting in approximate proportions of 85 percent by weight of caustic soda, 10 percent by weight of sodium chloride, 1 percent of sodium aluminate and 0.5 to 10 percent by weight of sodium cyanide.

5. A method of removing brown stains from various metals comprising immersing the metal workpiece in a molten salt bath composition comprising 75 to by weight of caustic alkali, '7 to 13% by weight of sodium chloride, 2 to 6% by weight of sodium fluoride, 0.5 to 5% by weight of sodium aluminate and 0.5 to 10% by weight of alkali cyanide. v

- HUGH G. WEBSTER,

REFERENCES CITED The following references are of record in the 

1. FOR ELECTROLYTICALLY CLEANING METAL, A COMPOSITION COMPRISING 75 TO 90 PERCENT BY WEIGHT OF CAUSTIC ALKALI, 7 TO 13 PERCENT BY WEIGHT OF SODIUM CHLORIDE, 0.5 TO 5 PERCENT BY WEIGHT OF SODIUM ALUMINATE AND 0.5 TO 10 PERCENT BY WEIGHT OF ALKALI CYANIDE. 